This research proposes to develop improved composite dental restorative materials based on organosilicon polymers. By proper selection of organosilicon monomers and optimization of composite formulation, the monomers are expected to enhance the bonding between the-filler and the polymer matrix and minimize the chemical degradation and water sorption. The major aim of the research is to develop composite materials based on organosilicon polymers which are highly durable and resistant to wear, and thus are suitable for use in posterior restorations as well as anterior restorations. Specifically, it is planned to use four high molecular weight siloxane monomers to prepare visible light-activated microfilled composites. The composites will be evaluated in vitro guided by ADA specification No. 27. Hardening time, hardness, diametral tensile strength, and water sorption of the composites will be determined and compared with those of BIS-GMA-based composites. The degree of conversion of the siloxane monomers will be determined by infrared spectroscopy, and correlated with their chemical structures and performance as the composite polymer matricies.